Reference Design Characteristics This document describes a high efficiency, rugged linear amplifier reference design for 2 meter amateur band (144 MHz -- 148 MHz) operation. Because of the ruggedness and low thermal resistance of the MRFE6VP61K25H transistor used in the design. the design can output high power even when operating into high VSWR. The amplifier can be biased for Class AB linear or Class C operation and is suitable for both analog and digital waveforms (AM/SSB or WSJT/FM/CW). · Frequency Band: 144--148 MHz · Output Power: &gt;1250 Watts CW · Supply Voltage: 50 Vdc · Power Gain (Typ): 26 dB · Class C Drain Efficiency (Min): &gt;78% · IMD @ 1 kW Output: &lt; --28.5 dB The MRFE6VP61K25H transistor used in this design is one of the devices in Freescale's RF power enhanced ruggedness 50 volt LDMOS product line. These devices, including the 600 watt MRFE6VP5600H and the 300 watt MRFE6VP6300H, are all specifically designed for 50 volt operation under harsh conditions.

MRFE6VP61K25H MRFE6VP61K25HS 2 Meter Amateur

144-148 MHz, 1250 W CW, 50 V 2 METER AMATEUR REFERENCE DESIGN

VDD BIAS M + M BIAS

VGG BIAS

RF INPUT

+

M M BIAS

RF OUTPUT

M = Match

VGG

VDD

2 METER AMATEUR REFERENCE DESIGN

This reference design is designed to demonstrate the RF performance characteristics of the MRFE6VP61K25H/HS devices operating in 144--148 MHz amateur radio band. The reference design shows two operational modes with different optimizations, VDD = 50 volts, IDQ = 2500 mA for Class AB linear operation or VDD = 43 volts, IDQ = 200 mA for Class C operation. own product or products. The reference design contains an easy--to--copy, fully functional amplifier design. It consists of &quot;no tune&quot; distributed element matching circuits designed to be as small as possible, and is designed to be used as a &quot;building block&quot; by our customers.

HEATSINKING

When operating this fixture it is critical that adequate heatsinking is provided for the device. Excessive heating of the device may prevent duplication of the included measurements and/or destruction of the device.

REFERENCE DESIGN LIBRARY TERMS AND CONDITIONS

Freescale is pleased to make this reference design available for your use in development and testing of your

Measurement is done using a CW (single tone) signal unless specified otherwise. Data was taken using an automated characterization system, ensuring repeatable measurements. The reference design was tuned with a trade--off between linearity and efficiency. Other tuning optimizations are possible.

The input circuit uses a 9/1 balun transformer with a prematch done by a series inductor and a shunt capacitor. The shunt capacitor is optional but is useful to center the input return loss (IRL). The input circuit return loss is always better than 10.5 dB, equivalent to a worst case VSWR of 1.8. The output circuit consists of a 4/1 transformer using two 4.7 lengths of 10 coaxial cable. It is also recommended that three DC blocks in parallel be used in order to lower the total equivalent series resistance (ESR) which is critical at this high power. The output balun is made from a 6.7 length of &quot;Sucoform 250&quot; 50 coaxial cable, and acts as a Pi match with 2 x 15 pF at the input and 5.6 pF at the output.

* PCB artwork for this reference design is available at http://freescale.com/RFindustrial &gt; Design Support &gt; Reference Designs or http://freescale.com/RFbroadcast &gt; Design Support &gt; Reference Designs.

IMD measurement was done using two signal generator with a tone spacing of 1 kHz. Quiescent current was set for 2.5 A under 50 volts with no RF signal at input. 2.5 A was choosen as a good compromise between gain, linearity and efficiency. In order to get optimal linearity, a thermal compensation circuit was used that tracks the quiescent current of the board over the temperature range (not shown on picture). Refer to Freescale's AN1643 RF LDMOS Power Modules for GSM Base Station Application: Optimum Biasing Circuit application note(1) or the VHF Broadcast reference design for more information.(2) The two--tone IMD values are referenced to the peak envelope power (PEP) and are spaced 1 kHz apart.

At the one kW level, second harmonic is --42 dBc, third harmonic is --32 dBc, and fourth harmonic is --37 dBc. To be used &quot;on the AIR&quot; this amplifier will likely need a filter to be compliant with local regulations. A diplexer could give better results than a simple low pass filter because harmonics are absorbed in a resistive load instead of being reflected to the transistor.

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Figure 11. Harmonics @ 1 kW

FREESCALE RF POWER 50 V TECHNICAL ADVANTAGES 50 V Drain Voltage

50 volt operation offers benefits over lower voltage operation because the output impedance of the device for the same output power is much greater, so the output match circuitry is simpler and has lower loss. IMD performance is better and supply current will also be lower than with low voltage operation. The reference fixture was designed with the market standard power supply, allowing the amplifier to utilize a standard 48 volt power supply (most are adjustable from 43 to 54 volts). Setting the gate bias voltage to around --4 volts will totally block the transistor even if the RF input signal is still there.

The enhanced electro--static discharge protection structure at the gate of the transistor is a Freescale innovation pioneered in the cellular infrastructure market that is incorporated into the 50 V LDMOS RF power product portfolios. This ESD structure can tolerate moderate reverse bias conditions applied to the gate lead up to --6 volts (see Figure 12). This allows Freescale transistors to be used in applications where the gate voltage needs to be set as low as --6 volts. This feature can dramatically simplify protection circuits, as it allows the transistor to be shut down because of high VSWR or PLL unlock without shutting down the drive power.

MRFE6VP61K25H is a very rugged part capable of handling 65:1 VSWR, provided thermal limits are not exceeded. It was designed for high mismatch applications, such as laser and plasma exciters, that under normal operation exhibit high VSWR values at startup and then come back to a more friendly impedance. In CW at high VSWR values and simultaneously at rated power, the limiting factor is the maximum DC power dissipation. VSWR protection that shuts down the gate voltage within 10 ms will protect the transistor effectively. The amplifier presented here was tested at full power with all phase angles with 10 ms pulsed 5% duty cycle without failure or degradation in RF performance. · Temperature rise (junction to case) = 219 Watts × 0.15°C/W = 32.8°C · TJ = Trise + Tcase = 63°C + 32.8°C = 95.8°C Utilizing the graph below which cacluates MTTF versus IDrain and TJ; IDrain = 28 A, MTTF for this example was 8000 years.

MTTF is defined as the mean time to failure of 50% of the device within a sample size, the primary factor in device reliability failure is due to electromigration. Once average operating condition for the applicatin is set, MTTF can be calculated using the Rth found on the offical Freescale data sheet. Example: If desired operating output power is 1000 watts, with 82% drain efficiency at 43 volts: · IDrain @ 1 kW 82% eff = 28.2 A · MRFE6VP61K25H Rth = 0.15°C/W, case temperature = 63°C · Dissipated power = 219 Watts

Figure 13. MTTF versus Junction Temperature There is an MTTF (Median--Time--To--Failure) calculator(3) available to assist the customers in estimating the MRFE6VP61K25H device reliability in terms of electromigration wear--out mechanism.

After one minute at 1 kW CW 44 volt supply at 80% efficiency, with no airflow on the top of the board, the output capacitor matching runs at 55°C, and the 10 coax section is around 90°C. After 5 minutes &quot;key down&quot; CW, the highest temperature is 113°C on the 10 coax section (Teflon cable is rated up to 200°C), output match capacitors do not show signs of overheating. If the board is run at levels higher than 1 kW CW or digital mode, airflow over the top side of the board could help to cool down coax and improve reliability.

As shown in Figure 14, the board was painted with black coating to correct for variations in emissivity

Technical documentation, including data sheets and application notes, for Freescale RF Power product can be found at: http://freescale.com/RFpower. Enter the applicable Document Number into &quot;Keyword&quot; search for quickest results.

The board drive level is very low and excessive drive level will destroy the transistor. If used with a transmitter, be careful with your power control as some transmitters have very high power spikes at startup due to a badly designed ALC. It is a better idea is to put a power attenuator ahead of this amplifier to protect against overdrive.

An Arlon TC350 PCB was chosen for its high thermal conductivity. Mounting is done on a copper heat spreader. Flatness under the transistor flange is critical; good flatness is mandatory for both RF and thermal performance. The transistor is mounted on the heat spreader using a thin layer of thermal compound. When using bolt--down mounting do not over--torque the part. Over tightening the fasteners can deform the transistor flange and degrade both the RF and thermal performance, as well as long term reliability. To reach optimum performance, the PCB must be soldered to the copper heat spreader. This is usually done using a hotplate and solder paste. It is critical that the soldering near the transistor and connectors is free of voids and is of high quality in to order to achieve best performance and reliability. Refer to Freescale's AN1617 Mounting Recommendations for Copper Tungsten Flanged Transistors application note for more information.(4)